1. Field of the Invention
The present invention relates to a method and apparatus for improving acknowledgement/negative-acknowledgement (ACK/NACK) bundling, and more particularly, to a method and apparatus for improving ACK/NACK bundling in a user equipment (UE) of a wireless communication system to correctly determine whether any PDCCH downlink assignment is missing, such that ACK or NACK can be transmitted.
2. Description of the Prior Art
Long Term Evolution wireless communications system (LTE system), an advanced high-speed wireless communications system established upon the 3G mobile telecommunications system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in Node B alone rather than in Node B and RNC (Radio Network Controller) respectively, so that the system structure becomes simple.
The LTE system supports Frequency Division Duplex (FDD) mode and Time Division Duplex (TDD) mode. The frame structure in the FDD mode is different from that in the TDD mode. In the FDD mode, a 10 ms long radio frame is divided into 10 sub-frames. Each sub-frame includes two slots, and each slot is 0.5 ms long. In the TDD mode, the 10 ms radio frame has two half-frames, each 5 ms long. Each half-frame includes four normal sub-frames and one special sub-frame, each 1 ms long. The special sub-frame includes three special slots: Downlink Pilot Time Slot (DwPTS), Guard Period (GP) and Uplink Pilot Time Slot (UpPTS). The DwPTS is used for transmitting downlink control signals, the UpPTS is used for transmitting uplink control signals, and the GP is a guard period between the DwPTS and the UpPTS. A total length of the three special slots is 1 ms long, and each length can be configured according to practical requirement of the network.
Compared to the FDD mode, the TDD mode supports different time allocation for uplink and downlink. That is, a ratio of uplink transmission time and downlink transmission time may not be 1:1. The allocation for uplink and downlink sub-frames can be adjusted according to transmission service types, so as to meet asymmetric uplink/downlink transmission requirement. For example, according to current specifications, six allocation ratios for uplink and downlink sub-frames are defined in the TDD mode, and are controlled by TDD UL/DL configurations 0-6 indicated by a higher layer. As for related definition of the TDD UL/DL configurations and the corresponding allocation ratios for uplink and downlink sub-frames, please refer to related specifications, which are not narrated herein.
For different TDD UL/DL configurations, the number of sub-frames for uplink transmission and downlink transmission are different. If the downlink sub-frames are more than the uplink sub-frames, feedbacks of multiple downlink transmissions can only be performed in one uplink sub-frame. In other words, a UE needs to transmit multiple ACK/NACKs corresponding to downlink packets being received in one uplink sub-frame, so as to support Hybrid Automatic Repeat reQuest (HARQ) operation. In this case, in order to reduce an amount of bits transmitted by the uplink sub-frame, a technique of ACK/NACK bundling is introduced in the prior art, which utilizes one ACK/NACK signal to complete HARQ feedbacks of multiple downlink packets being received, so as to solve the problem resulted from asymmetric uplink/downlink transmission. Please note that, in the following description, the uplink sub-frame for ACK/NACK transmission is called a common sub-frame, while a set of downlink sub-frames corresponding to the common sub-frame are called bundled sub-frames.
Generally, ACK/NACK bundling generates the HARQ feedback in the common uplink sub-frame by performing a logical AND operation for the ACK/NACK bits of the downlink packets received within the bundled sub-frames. For example, if all the HARQ feedbacks of the downlink packets received within the bundled sub-frames are ACKs, the UE would transmit an ACK in the common uplink sub-frame, so as to acknowledge reception of all the received packets. On the contrary, if any HARQ feedback of the downlink packets received within the bundled sub-frames is a NACK, the UE would generate a NACK in the common uplink sub-frame to request retransmission of all the received packets.
Besides, for each uplink sub-frame or downlink sub-frame, the UE shall monitor a Physical Downlink Control Channel (PDCCH) to detect whether there is any uplink grant or downlink assignment dynamically scheduled by the network, so as to perform a Physical Uplink Shared Channel (PUSCH) transmission in a corresponding uplink sub-frame or receive a Physical Downlink Shared Channel (PDSCH) transmission in a corresponding downlink sub-frame accordingly. Therefore, if a downlink assignment dynamically scheduled by the network is missed, since the UE does not know there is a downlink packet needed to be received in a corresponding downlink sub-frame, the UE would not generate a corresponding HARQ feedback as well. In this case, since the HARQ feedback transmitted in the common sub-frame are generated according to the HARQ feedbacks of the received packets, the network cannot know the UE fails to receive the downlink packet due to missing the downlink assignment. Therefore, the network would not perform any packet retransmission, causing the downlink packet is lost by the physical layer.
In order to avoid packet loss due to a downlink assignment miss, the prior art adds a 2 bit Downlink Assignment Index (DAI) field in a PDCCH signaling. The DAI denotes a number of downlink assignments the UE shall receive, such that the UE can correctly transmit ACK or NACK by the ACK/NACK bundling.
For different PDCCH formats, the DAI field has different meanings. For a PDCCH with Downlink Control Information (DCI) format 0, e.g. a PDCCH carrying an uplink grant allocated to the common sub-frame, the DAI field denotes a total number of downlink sub-frames with PDSCH transmissions within the bundled sub-frames, and is denoted by a parameter V_UL_DAI in the UE. In other words, the value of the parameter V_UL_DAI includes dynamic scheduled PDSCH transmissions (i.e. PDSCH transmission with corresponding PDCCH) and pre-defined PDSCH transmissions (i.e. PDSCH transmission without corresponding PDCCH), such as Semi-Persistent Scheduling (SPS) transmission.
For a PDCCH with DCI format 1, 1A, 1B, 1D, 2 and 2A, e.g. a PDCCH carrying a downlink assignment allocated to the bundled sub-frames, the DAI field denotes an accumulative number of PDSCH transmissions with corresponding PDCCH up to a current downlink sub-frame within the bundled sub-frames, and is denoted by a parameter V_DL_DAI in the UE.
In addition, the UE maintains a parameter U_DAI and a parameter N_SPS. The parameter U_DAI records a number of dynamic scheduled PDSCH transmissions up to the current downlink sub-frame within the bundled sub-frames. The parameter N_SPS records a number of pre-defined PDSCH transmissions up to the current downlink sub-frame within the bundled sub-frames.
Therefore, whenever the UE detects a PDCCH signaling, the UE can determine whether any downlink assignment is missing by comparing the DAI carried by the PDCCH signaling with the parameters U_DAI and N_SPS maintained by the UE itself. Besides, when the UE performs uplink transmission in the common sub-frame, the UE can further determine a number of downlink sub-frames that generate HARQ feedbacks within the bundled downlink sub-frames, which is denoted by a parameter N_bundled.
Please note that the DAI field carried by PDCCH only has two bits. Therefore, in the current specifications, the parameter V_UL_DAI and V_DL_DAI are represented by values of 1-4. However, since the number of downlink sub-frames within the bundled sub-frames may be 0-9, a modulo operation is performed to correlate the parameter V_UL_DA I and V_DL_DAI with the number of downlink sub-frames. For example, the value “1” represents 1, 5 or 9 downlink sub-frames, the value “2” represents 2 or 6 downlink sub-frames, the value “3” represents 3 or 7 downlink sub-frames, and the value “4” represents 0, 4 or 8 downlink sub-frames.
According to the current specifications, for the case that the UE does not perform uplink transmission in the common sub-frame, since the UE does not receive any uplink grant allocated to the common sub-frame, the UE can only rely on the DAI carried in the received PDCCH signaling for downlink assignment (i.e. V_DL_DAI) to determine whether any downlink assignment is missing. If the parameter V_DL_DAI does not conform to the parameter U— DAI, i.e. VDAIDL≠(UDAI−1)mod 4+1, the UE determines that at least one downlink assignment is missing.
For the case that the UE performs uplink transmission in the common sub-frame and the uplink transmission is adjusted according to a PDCCH with DCI format 0 (i.e. the UE has received an uplink grant allocated to the common sub-frame by the network), the UE can determine whether any downlink assignment is missing according to the DAI carried in the received PDCCH signaling for uplink grant (i.e. V_UL_DAI). If the parameter V_UL_DAI does not conform to a sum of the parameter U_DAI and the parameter N_SPS, i.e. VDAIUL≠(UDAI+NSPS−1)mod 4+1, then the UE determines that at least one downlink assignment is missing and generates a NACK in the common sub-frame. Besides, the UE sets the number of downlink sub-frames that generate the HARQ feedbacks within the bundled sub-frames (i.e. the parameter N_bundled) as the value of the parameter V_UL_DAI. If the sum of the parameter U_DAI and the parameter N_SPS is 0 and the parameter V_UL_DAI is 4, i.e. there is no PDSCH transmission within the bundled sub-frames, then the UE shall not transmit any ACK or NACK.
For the case that the UE performs uplink transmission in the common sub-frame and the uplink transmission is not adjusted according to a PDCCH with DCI format 0, i.e. the UE performs the PUSCH transmission according to a pre-defined uplink grant, the UE can only determine whether any downlink assignment is missing according to the DAI carried in the received PDCCH signaling for downlink assignment (i.e. V_DL_DAI). If the parameter V_DL_DAI does not conform to the parameter U_DAI, i.e. VDAIDL≠(UDAI−1)mod 4+1, the UE determines least one downlink assignment is missing, and the UE shall generate a NACK in the common sub-frame. Besides, the UE sets the number of downlink sub-frames that generate the HARQ feedbacks within the bundled sub-frames (i.e. N_bundled) as the sum of the parameter U_DAI and the parameter N_SPS. If the sum of the parameter U_DAI and the parameter N_SPS is 0, i.e. there is no PDSCH transmission within the bundled sub-frames, then the UE shall not transmit any ACK or NACK.
However, according to the current specifications, for TDD UL/DL configuration 0, the PDCCH with DCI format 0 (i.e. the PDCCH carrying an uplink grant) does not have a DAI field. Therefore, for a scenario that TDD UL/DL configuration is 0 and the PDCCH with DCI format 0 is received, the UE can neither determine whether any downlink assignment is missing nor determine whether to transmit ACK/NACK in the common sub-frame. In addition, the UE cannot correctly set the number of downlink sub-frames that generate the HARQ feedbacks within the bundled sub-frames (i.e. N_bundled) as well.